Design of propeller driven watercraft, including surface craft and submarines, involves a number of well known compromises involving propeller size, placement of the engine, and hull shape, to name but a few of the issues. In addition, the column of thrust fluid propelled by a single propeller rotates. Rotation of the thrust fluid does not produce thrust, though is required in order to move the thrust fluid backward (which does produce thrust). Thrust fluid rotation can be eliminated or at least balanced through the use of two counter-rotating propellers, though this results in twice the propeller surface area and (typically) twice as much drive train complexity, which reduces efficiency. In addition, efficient propeller-driven watercraft achieve roughly 0.7 on a graph of propulsive efficiency and thrust coefficient, and, even then, only in a narrow range of speeds. See, for example, FIG. 31, which is a graph from “Hydrodynamic Flow Control in Marine Mammals”, by Frank E. Fish, Laurens E. Howie, and Mark M. Murray, presented in the symposium, “Going with the Flow: Ecomorphological Variation across Aquatic Flow Regimes”, presented at the annual meeting of the Society for Integrative and Comparative Biology, Jan. 2-6, 2008, at San Antonio, Tex., United States. The efficiency curve is approximately an inverted parabola. Travel faster or slower than the speed where peak efficiency occurs, and the efficiency of the propeller-driven craft drops off rapidly.
In addition, propeller driven watercraft typically have a drive-shaft which, when the engine is inboard, penetrates the hull and creates the need for a drive-shaft seal (outboard motors have a severe bend in the drive-shaft, which reduces efficiency relative to inboard motors). Drive-shaft seals create friction, require maintenance, and introduce added mechanical complexity (such as a bilge pump).
Electric motors can be utilized which are flooded with a liquid and which thereby reduce the internal-external pressure differential on the drive-shaft seal. Such motors are sometimes found in submarines; however, such motors experience greater friction because the rotor rotates in a liquid, rather than in air, and maintenance is more complex.
In contrast to propellers, fins—marine mammals and fish—have an efficiency/thrust coefficient of approximately 0.8 and the efficiency curve is very flat. See, again, FIG. 31. Traveling faster or slower than the speed of peak efficiency results in only a modest change in efficiency. While vortexes are present in the thrust fluid propelled by a fin, unlike rotation of the column of thrust fluid coming off of a propeller, the vortexes behind a fin counter-rotate. The vortexes form a “reverse von Karman street” pattern, in which downstream vortices, as they spin and release energy over time, appear to pull upstream vortices further downstream, scavenging energy and contributing to overall thrust.
However, connecting a motor to a fin is a complex problem, particularly in a marine environment. Many fin-based propulsion systems have been designed and built, some of which produce a fish-like motion. Often, such systems have tens, hundreds, or even thousands of intricately machined parts with tight tolerances. Often, such systems have multiple moving bearings which are exposed to or which need to be sealed away from water by a “wet” seal (which attempts to seal the moving part or its bearings from water). Often, the bearings in such craft experience asymmetric loads, first on one side and then on the other. Some of such systems rely on exotic, expensive, and fragile materials, such as materials which contract or expand in an electric field.
The sheer number of parts, parts which move, seals, and asymmetrically loaded bearings reduce the efficiency of such systems, increase manufacturing costs, and decrease reliability, rendering most fin-based watecraft propulsion systems impractical for commercial use.
Needed is an inexpensive, efficient, robust, fin-based propulsion system.
Disclosed is an efficient fin-based propulsion system with only one directly powered component which, in some embodiments, is entirely sealed.